Natural Assemblages Research Articles

Bacterial surface interactions with organic colloidal particles: Nanoscale hotspots of organic matter in the ocean.

Colloidal particles constitute a substantial fraction of organic matter in the global ocean and an abundant component of the organic matter interacting with bacterial surfaces. Using E. coli ribosomes as model colloidal particles, we applied high-resolution atomic force microscopy to probe bacterial surface interactions with organic colloids to investigate particle attachment and relevant surface features. We observed the formation of ribosome films associating with marine bacteria isolates and natural seawater assemblages, and that bacteria readily utilized the added ribosomes as growth substrate. In exposure experiments ribosomes directly attached onto bacterial surfaces as 40-200 nm clusters and patches of individual particles. We found that certain bacterial cells expressed surface corrugations that range from 50-100 nm in size, and 20 nm deep. Furthermore, our AFM studies revealed surface pits in select bacteria that range between 50-300 nm in width, and 10-50 nm in depth. Our findings suggest novel adaptive strategies of pelagic marine bacteria for colloid capture and utilization as nutrients, as well as storage as nanoscale hotspots of DOM.

Testing the use of the silica deposition fluorescent probe PDMPO to estimate in situ growth rates of diatoms

AbstractThe fluorophore [2‐(4‐pyridyl)‐5{[4‐dimethylaminoethyl‐aminocarbamoyl‐methoxy]phenyl}oxazole], in short PDMPO, is incorporated in newly polymerized silica in diatom frustules and thereby provides a tool to estimate Si uptake, study diatom cell cycles but also determine mortality‐independent abundance‐based species specific‐growth rates in cultures and natural assemblages. In this study, the theoretical framework and applicability of the PDMPO staining technique to estimate diatom species specific‐growth rates were investigated. Three common polar diatom species, Pseudo‐nitzschia subcurvata, Chaetoceros simplex, and Thalassiosira sp., chosen in order to cover a broad range of species specific frustule and life‐cycle characteristics, were incubated over 24 h in control (no PDMPO) and with 0.125 and 0.6 μM PDMPO addition, respectively. Results indicate that specific‐growth rates of the species tested were not affected in both treatments with PDMPO addition. The specific‐growth rate estimates based on the PDMPO staining patterns (μPDMPO) were comparable and more robust than growth rates estimated from the changes in cell concentrations (μcc). This technique also allowed to investigate and highlight the importance of the illumination cycle (light and dark phases) on cell division in diatoms.

A Multivariable Empirical Algorithm for Estimating Particulate Organic Carbon Concentration in Marine Environments From Optical Backscattering and Chlorophyll-a Measurements

Accurate estimates of the oceanic particulate organic carbon concentration (POC) from optical measurements have remained challenging because interactions between light and natural assemblages of marine particles are complex, depending on particle concentration, composition, and size distribution. In particular, the applicability of a single relationship between POC and the spectral particulate backscattering coefficient bbp(λ) across diverse oceanic environments is subject to high uncertainties because of the variable nature of particulate assemblages. These relationships have nevertheless been widely used to estimate oceanic POC using, for example, in situ measurements of bbp from Biogeochemical (BGC)-Argo floats. Despite these challenges, such an in situbased approach to estimate POC remains scientifically attractive in view of the expanding global-scale observations with the BGC-Argo array of profiling floats equipped with optical sensors. In the current study, we describe an improved empirical approach to estimate POC which takes advantage of simultaneous measurements of bbp and chlorophyll-a fluorescence to better account for the effects of variable particle composition on the relationship between POC and bbp. We formulated multivariable regression models using a dataset of field measurements of POC, bbp, and chlorophyll-a concentration (Chla), including surface and subsurface water samples from the Atlantic, Pacific, Arctic, and Southern Oceans. The analysis of this dataset of diverse seawater samples demonstrates that the use of bbp and an additional independent variable related to particle composition involving both bbp and Chla leads to notable improvements in POC estimations compared with a typical univariate regression model based on bbp alone. These multivariable algorithms are expected to be particularly useful for estimating POC with measurements from autonomous BGC-Argo floats operating in diverse oceanic environments. We demonstrate example results from the multivariable algorithm applied to depth-resolved vertical measurements from BGC-Argo floats surveying the Labrador Sea.

Thermodynamic modeling of melt addition to peridotite: Implications for the refertilization of the non-cratonic continental mantle lithosphere

In a classic model of evolution of the non-cratonic continental mantle lithosphere, harzburgites represent the refractory (<5% clinopyroxene) residues of high degrees of partial melting of fertile mantle, while lherzolites (>5% clinopyroxene) represent residues of lesser degrees of partial melting. However, partial melting is not the only process that could explain the peridotite compositional variability that ranges from fertile (>2 wt% Al2O3, <45 wt% MgO) to refractory (<2 wt% Al2O3, >45 wt% MgO). In the refertilization process, harzburgite is a refractory protolith (potentially previously formed by partial melting of a fertile mantle) that undergoes reactive percolation of silicate melts derived from the underlying asthenosphere, resulting in the crystallization of a new generation of minerals (mostly clinopyroxene). A simple but critical first step towards understanding the refertilization process is to examine how modal and major element compositions evolve as melts are added to peridotites. Here we use a thermodynamically-constrained two-component mixing model to independently evaluate the roles of five different parameters: pressure, temperature, redox conditions, and compositions of the initial peridotite and the added basaltic melt (hereafter referred to P-T-fO2-Xπ-Xmelt), during melt addition. We compare the results with observed suites of peridotites. The main observations are as follows: (1) the produced model is consistent with the global peridotite database, and (2) T, fO2 and small variations of pressure have almost no impact on the evolution of the system. In contrast, the mineralogy of the percolated harzburgite has a substantial effect on the variation of the modal proportions. The parameter with the most significant impact is Xmelt, which is directly linked to the geodynamic context and melting conditions. This parameter directly controls the refertilization reaction and so, the phase proportions and the bulk-rock composition. Elements that partition preferentially in the melt phase (e.g., Na) display depletions in natural assemblages that are stronger than those predicted from the simple mixing model, consistent with the fact that the natural process occurs in an open system, and that reactive percolation likely results in incompatible element enrichment in the associated melt. Our results corroborate the suggestion that most of the spectrum of compositional variability observed in lithospheric mantle peridotites can be explained by the impregnation of primitive silicate melt in refractory harzburgites.

Modeling of Magnetic Anisotropy in Electrical Steel Sheet by Means of Cumulative Distribution Functions of Gaussians

This article proposes a methodology for modeling magnetic anisotropy in ferromagnetic materials. This approach was initially used to represent the isothermal remanent magnetization (IRM) acquired by natural mineral assemblages and based on the use of the cumulative distribution function (CDF) of a probability density function (PDF). For the purpose of this work, the anisotropy of electrical steel sheets is modeled by means of CDFs of Gaussians. The methodology is validated by comparisons between numerical and experimental data obtained from an Epstein frame and a Rotational Single Sheet Tester. The outcomes of the study are twofold: first, the parameters defining the CDF follow a polynomial trendline, and second, they can easily be calculated from a limited amount of experimental data which is a key point from an engineering point of view.

Experimental Investigation of Trachydacite Magma Storage Prior to the 1257 Eruption of Mt Samalas

Abstract The caldera-forming Samalas eruption of the Samalas–Rinjani volcanic complex on Lombok, Indonesia, in 1257 CE ranks as one of the most explosive and sulphur-rich eruptions of the past thousand years. Along with other significant events (e.g. Tambora 1815, El Chichón 1982), Samalas 1257 forms a class of powerful eruptions of magmas of intermediate alkaline composition, which remain relatively understudied. This study aims to better understand the pre-eruptive magma reservoir conditions and volatile storage capacities of intermediate alkaline systems, using the 1257 Samalas eruption as a case study. Systematic partial-equilibrium experiments were run to constrain potential magma reservoir conditions (25–200 MPa and 850–1000°C) by examining changes in the chemistry and textures of experimental glass and mineral products. Natural trachydacite pumice from the 1257 Samalas eruption was used as the experimental charge for a series of water-saturated experiments run in cold-seal pressure vessels, buffered at an oxygen fugacity ƒO2 of NNO/NNO + 1 log units. Assessing the match between natural and experimental glass compositions and mineral phases and textures reveals that pre-eruptive magma reservoir conditions were between 875°C and 930°C and ca. 100–150 MPa (4.5 ± 1km depth). Breakdown of experimental amphibole at pressures below 75 MPa, and plagioclase instability at 950°C/100 MPa, provide the strongest constraints on phase stabilities that are consistent with the magma storage region. In the observed temperature and pressure range, the natural compositions of plagioclase, amphibole and orthopyroxene are replicated in experimentally precipitated rims. Clinopyroxene and high anorthite plagioclase (An&amp;gt;64) in the natural mineral assemblage could not be replicated in the experiments, implying that these compositions most likely did not grow in the shallow pre-eruptive reservoir of Mt Samalas. The pre-eruptive storage depths of trachydacite magma are significantly shallower than the postulated magma storage at the currently active basaltic-andesite magmatic system at Rinjani, and thus monitoring magma storage depth may be a useful indicator of changing magma composition in the future. Our findings show that highly explosive VEI 7 eruptions of intermediate alkaline magma can be fed from a relatively limited range of storage pressures (100–150 MPa), suggesting accumulation of magma in one place, rather than the rapid extraction of magma from a vertically extensive, transcrustal magma system.

Shiny bronze in glassy matter: an inconspicuous piece of slag from the Bronze Age mining site of Mušiston (Tajikistan) and its significance for the development of tin metallurgy in Central Asia

This paper aims at contributing to a better understanding of the beginnings of tin and bronze metallurgy in Central Asia by investigating a hitherto unique piece of a bronze slag. The object was originally discovered as a stray find only 4 km away from the large copper-tin deposit of Mušiston in Tajikistan. It contains many prills of bronze and copper as well as small charcoal particles. Radiocarbon dating of the charcoal places the slag in a period between 1900 and 1400 BCE and thus in the Late Bronze Age of the region. This date coincides with radiocarbon dates of relics from underground galleries of the Mušiston deposit. Chemical and microscopic examination demonstrated the slag to be a relic of a co-smelting process, in which a natural assemblage of tin and copper minerals was smelted simultaneously. Both the chemical and the tin and copper isotope compositions clearly link the slag to the nearby polymetallic ores from Mušiston, of which an extensive dataset is presented. The artefact’s lead isotope ratios and increased iron concentration in turn indicate intentional fluxing of the original ore charge with iron-dominated ores. These results are the first tangible evidence of a smelting process of tin ores in the entire region and therefore add a new dimension to the findings from previous mining archaeological investigations. At the same time, the results give significant information about the smelting process of secondary polymetallic ores from Mušiston and help in assessing the scientific data of Bronze Age bronze artefacts from Central Asia.

Diversity of Nearctic Dragonflies and Damselflies (Odonata)

Rarely have studies assessed Odonata diversity for the entire Nearctic realm by including Canada, the United States, and Mexico. For the first time, we explored Odonata diversity in this region according to a definition of natural community assemblages and generated species distribution models (SDMs). Species occurrence data were assembled by reviewing databases of specimens held by significant Odonata repositories and through an extensive search of literature references. Species were categorized as forest-dependent or non-forest-dependent, as lentic or lotic-dependent, and according to conservation status. Predicted distributions were stacked for all species across their entire ranges, including areas outside of the Nearctic. Species richness and corrected weighted endemism (CWE) were then calculated for each grid cell. We found a pattern of greater species richness in the eastern portion of the Nearctic, which can be explained by the higher aquatic habitat diversity at micro and macroscales east of the Rocky Mountains, promoting niche partitioning and specialization. In the Nearctic region, the southeastern US has the highest number of endemic species of dragonflies and damselflies; this degree of endemism is likely due to glacial refuges providing a foundation for the evolution of a rich and unique biota.

Phytoplankton growth and community shift over a short-term high-CO2 simulation experiment from the southwestern shelf of India, Eastern Arabian Sea(summer monsoon).

The southwestern shelf water of India (eastern Arabian Sea) experiences high seasonality. This area is one of the understudied regions in terms of phytoplankton response to the projected ocean acidification, particularly, during the summer monsoon when phytoplankton abundance is high. Here we present the results of a short-term simulated ocean acidification experiment (ambient CO2 424 µatm; high CO2, 843, 1138 µatm) on the natural phytoplankton assemblages conducted onboard (R. V. Sindhu Sadhana) during the summer monsoon (Aug 2017). Among the dissolved inorganic nutrients, dissolved silicate (DSi) and nitrate + nitrite levels were quite low (< 2µM). Phytoplankton biomass did not show any net enhancement after the incubation in any treatment. Both marker pigment analysis and microscopy revealed the dominance of diatoms in the phytoplankton community, and a significant restructuring was noticed over the experimental period. Divinyl chlorophylla (DVChla) containing picocyanobacteria and 19'-hexanoyloxyfucoxanthin (19'HF) containing prymnesiophytes did not show any noticeable change in response to CO2 enrichment. A CO2-induced positive growth response was noticed in some diatoms (Guinardia flaccida, Cylindrotheca closterium, and Pseudo-nitzschia sp.) and dinoflagellates (Protoperidinium sp. and Peridinium sp.) indicating their efficiency to quickly acclimatize at elevated CO2 levels. This is important to note that the positive growth response of toxigenic pennate diatoms like Pseudo-nitzschia as well as a few dinoflagellates at elevated CO2 levels can be expected in the future-ocean scenario. The proliferation of such non-palatable phytoplankton may impact grazing, the food chain, and carbon cycling in this region.

Detangling ecosystem services: Open-field manipulation of soil-dwelling microarthropods provides new opportunities to investigate their effects on nitrogen cycling.

Soil microarthropods have a pivotal role in soil nitrogen cycling in that they affect microbial decomposers. A high abundance of microarthropods may increase the mobility of inorganic nitrogen ions in the soil, mainly in nitrogen‐limited habitats. However, it is difficult to study ecological processes with small‐sized, soil‐dwelling arthropods. The effects of soil microarthropods on nitrogen cycling have mainly been studied in laboratory microcosm experiments. Therefore, we face many practical issues in investigating these effects under field conditions that remain to be resolved.We developed an open‐field mesocosm setup with growing plants. In a two‐part experiment, spring wheat and grass species were grown in chernozem and sandy soils. Leached ammonium and nitrate ions were measured with percolation lysimeters. Half of the mesocosms included natural assemblages, and the other half included less abundant Acari and Collembola assemblages. The application of nitrogen fertilization assured differences in nitrogen sources.We found a large difference in ammonium and nitrate leaching between the two soil types. In chernozem soil, the leached ion concentrations were higher in mesocosms with more abundant mite and springtail assemblages. The expected patterns were less pronounced in sandy soil. Adding nitrogen fertilizer did not modify the effects of soil microarthropods.Open‐field mesocosms are promising for studying the role of soil‐dwelling mesofauna in ecological processes. We solved the problem of keeping mesofauna abundance lower in treated plots than that in control plots. Plants successfully grew in our semi‐closed systems with functioning percolation lysimeters. The use of the equipment in the experiments in this study helped reveal that the role of soil‐dwelling microarthropods in nitrogen cycling depends on the soil type and not on the application of nitrogen fertilizer.

Molecular phylogeny of foliose Halymenia and Austroepiphloea (Halymeniaceae, Rhodophyta) from the Indo-Pacific, with the description of Halymenia taiwanensis sp. nov.

ABSTRACT Many species of Halymenia from the Indo-Pacific have been described in the past decade, but their phylogenetic relationships are not well discussed. In this study, we inferred these relationships for the available species of Halymenia with an emphasis on the foliose species from the western Pacific Ocean and Western Australia based on rbcL and nuclear-encoded LSU rDNA sequence analyses. Our analyses show that most foliose Halymenia from the Indo-Pacific are clustered in a natural assemblage that also includes a new species (Halymenia taiwanensis sp. nov.) found in northern Taiwan, as well as the monospecific genus Austroepiphloea (single species A. bullosa) from Western Australia. The architecture of carpogonial branch (composed of a two-celled carpogonial branch and two orders of ampullar filaments, and a basal, nutritive cellular cluster) and auxiliary cell ampullae in Halymenia taiwanensis is similar to that found in the generitype H. floresii. We therefore propose the new combination Halymenia bullosa comb. nov. (basionym Schizymenia. bullosa), a species closely related to H. taiwanensis both genetically and morphologically. However, H. taiwanensis can be separated from H. bullosa by its thinner blades and surface bladelets, and by lacking a long cartilaginous stipe. Based on the rbcL phylogeny, most foliose Halymenia are seemingly more range-restricted than previously thought, except for a few species that are shown to have a wide distribution in the Western Pacific and Indian Oceans. In addition, H. dilatata, a species widely recorded in the Western Pacific Ocean, may include cryptic species and requires further investigation.

Copper chloride (II) effect on the composition and structure of marine microphytobenthic communities.

To assess the temporary effects of the increased copper ion inflow on estuarine microphytobenthic communities, ecotoxicological tests were conducted using natural microphytobenthic assemblages obtained from an artificial substratum exposed to the waters of the southern Baltic Sea (Gulf of Gdańsk). The applied copper ion concentrations reflected permitted copper values established for waters of a good ecological status (2·10-5g Cu·dm-3), and the maximum copper concentrations which, according to the current environmental regulations, are allowed to be discharged into the environment (2·10-3g Cu·dm-3).In the studied communities, diverse responses of single species to CuCl2 exposure were recorded, including both growth inhibition and stimulatory effects as well. Despite the shift in the community composition and structure, total cell number remained at a similar level. The results of our investigations suggest that microphytobenthic assemblages are resistant to CuCl2 which is facilitated by the shift in the community composition resulting from the increasing cell number of copper tolerant species.

Repression: Rethinking the Relationship between Creator and the Creation

Psychoanalysis is one of the most prominent literary theories used in modern interpretations, to explore the deeper meanings of a text and its characters. The purpose of psychoanalysis is to unveil a person's repressed emotions and experiences, making their unconscious conscious. Psychoanalysis, on the other hand, aids in knowing more about the natural assemblage of a writer's personality from birth to becoming an author. Thus, this paper aims to identify the conglomeration of the author's collected emotions and their representation in the chosen novel. The paper's finding reveals that Vegas is the place that emoted the author's repressed memories that have turned into a story. Therefore, an attempt is made to analyze the characters' actions and bridge the fictional characters and the author's personal experience using Freud's tripartite hypothesis on human mind structure: Id, Ego, and Superego

Nutrient Control of Phytoplankton Abundance and Biomass, and Microplankton Assemblage Structure in the Lower Columbia River (Vancouver, Washington, USA)

Nutrient limitation of phytoplankton is common but by no means universal in large temperate rivers. Previous field studies in the Columbia River, USA, are suggestive of nutrient limitations of phytoplankton, especially during summer, but this has never been tested experimentally. We therefore undertook monthly 5-day nutrient amendment incubation experiments from May–September 2018 using Columbia River water collected at Vancouver, Washington, USA. We compared replicate treatment bottles containing natural microplankton assemblages and amended nutrients (NO3, PO4 and SiO4 in combination) with replicate control bottles containing natural microplankton assemblages and ambient nutrients. Phytoplankton abundance and biomass were compared between treatments and controls on each day of each experiment, and microplankton assemblage structure was evaluated using Permutational Multivariate Analysis of Variance and Non-Metric Multi-Dimensional Scaling ordination on Day 0 (ambient) and Day 5 of each experiment. Nutrient amendment significantly affected phytoplankton abundance and biomass, particularly in June–August, although this varied between taxa (e.g., cyanobacteria, dinoflagellates, flagellates and ciliates showed more frequent positive responses than chlorophytes and diatoms did). Abundance-based microplankton assemblage structure was significantly correlated with PO4, SiO4 and NO3 concentrations, and BIOENV procedure in R revealed that the best subset of explanatory variables included SiO4 and NO3 concentrations. Biomass-based assemblage structure was significantly correlated with SiO4 and NO3, although BIOENV explanatory variables included only SiO4. These results are suggestive of summertime nutrient control of phytoplankton abundance and biomass, as well as microplankton composition, in the lower Columbia River, at least during some months. Since eutrophication is increasing in the watershed, this could have important implications for higher level consumers (e.g., zooplankton and out-migrating juvenile salmon).

Benthic fluxes of dissolved silica are an important component of the marine Si cycle in the coastal zone

Spatial and seasonal changes in benthic fluxes of dissolved silica (DSi) across the coastal zones were investigated in the southern Baltic Sea. Measurements were performed using ex situ incubations of sediment cores with natural benthic assemblages. Obtained benthic fluxes ranged from the uptake of −1.11 mmol m−2 d−1 in summer to a release of 6.79 mmol m−2 d−1 in autumn, while in situ concentrations in the bottom water were the lowest in autumn (1 μmol L−1) and the highest in winter (up to 58 μmol L−1). Sediments with high mud content had the highest fluxes in autumn, intermediate in spring, and the lowest in winter whereas no clear seasonal patterns were detected for sandy sediments. Generalized Linear Models indicated that in shallow enclosed areas biological factors, particularly presence of Chironomidae larvae, explained DSi fluxes variation while in open areas the environmental factors, such as organic matter quantity and quality, were key explanatory variables. In all studied environments (enclosed lagoon, open bay, and open coastal zone) DSi benthic fluxes represent an important component of the marine Si cycle. Further, the total yearly load of DSi from two major rivers (Vistula and Oder) and from the sediments, to the southern Baltic Sea reaches 258 kt y−1 of which benthic fluxes may constitute up to 34%.

Chemotaxis shapes the microscale organization of the ocean's microbiome.

The capacity of planktonic marine microorganisms to actively seek out and exploit microscale chemical hotspots has been widely theorized to affect ocean-basin scale biogeochemistry1-3, but has never been examined comprehensively in situ among natural microbial communities. Here, using a field-based microfluidic platform to quantify the behavioural responses of marine bacteria and archaea, we observed significant levels of chemotaxis towards microscale hotspots of phytoplankton-derived dissolved organic matter (DOM) at a coastal field site across multiple deployments, spanning several months. Microscale metagenomics revealed that a wide diversity of marine prokaryotes, spanning 27 bacterial and 2 archaeal phyla, displayed chemotaxis towards microscale patches of DOM derived from ten globally distributed phytoplankton species. The distinct DOM composition of each phytoplankton species attracted phylogenetically and functionally discrete populations of bacteria and archaea, with 54% of chemotactic prokaryotes displaying highly specific responses to the DOM derived from only one or two phytoplankton species. Prokaryotes exhibiting chemotaxis towards phytoplankton-derived compounds were significantly enriched in the capacity to transport and metabolize specific phytoplankton-derived chemicals, and displayed enrichment in functions conducive to symbiotic relationships, including genes involved in the production of siderophores, B vitamins and growth-promoting hormones. Our findings demonstrate that the swimming behaviour of natural prokaryotic assemblages is governed by specific chemical cues, which dictate important biogeochemical transformation processes and the establishment of ecological interactions that structure the base of the marine food web.

Assessing the potential of invertebrate natural enemies of insect pests inhabiting Miscanthus x giganteus shelterbelts in pasture

ABSTRACT Miscanthus x giganteus (Miscanthus) grass shelterbelts can deliver multiple ecosystem services on New Zealand commercial dairy farms. However, there has been little investigation into how these shelterbelts contribute to insect pest management. Here, on a Canterbury dairy farm, we investigated what generalist surface-dwelling invertebrate natural enemies of insect pests inhabit three separate Miscanthus shelterbelts compared to three unmanipulated field margin plots. The potential contribution of these natural enemy assemblages to future biological control was also investigated. To measure this, live moth egg baits with associated pitfall trapping were deployed in autumn, winter and spring of 2015. Miscanthus and the field margin plots were found to have similar potential natural enemy richness but differed in their community composition. The potential predation rate of pests in Miscanthus was 85% higher than in the field margin. Infrared video and Sanger sequencing confirmed that the harvestman Phalangium opilio and the slug Deroceras reticulatum consumed the egg baits in Miscanthus. Conversely, no bait-consuming invertebrates were identified in the field margin. These preliminary results indicate potential natural enemies inhabit Miscanthus and may consume insect pest eggs on the soil-surface. However, to achieve anyinsect pest suppression, further habitat manipulation would be required.

Summer dynamics drive the microbial response to carbon and nutrient additions in a high‐altitude lake

AbstractThe predicted increase in allochthonous dissolved organic carbon (DOC) in high‐altitude lakes is expected to alter the phytoplankton–bacterioplankton relationship. However, few studies address the influence of summer phytoplankton dynamics on microbial responses to DOC additions. We sampled natural plankton assemblages during two contrasting periods of summer in a high‐altitude lake in the French Alps and subjected them to glucose and nutrient enrichments under two light conditions (dark or light) and two temperature conditions (10°C or 18°C). Our results indicate that glucose use by bacteria differs over the summer, depending on the availability of autochthonous DOC and the nutrient limitation. Glucose was consumed by bacteria more in early summer; however, biomass increased with glucose addition more in late summer than in early summer. This pattern arose from the greater availability of phytoplankton‐derived DOC in late summer, reducing the need for alternative carbon sources in late summer, when phytoplankton biomass was high. Mixotrophic taxa were stimulated after glucose additions both in early summer and in late summer. We found greater competition between bacteria and phytoplankton in late summer after glucose addition, linked to the summer nutrient limitation pattern. Our study thus highlights a differential response depending on the timing of summer DOC inputs. The global changes forecast for the French Alps should increase heterotrophic and mixotrophic processes in planktonic communities of shallow high‐altitude lakes with vegetated catchments. This experimental study provides insights that will be useful in predicting ecological trajectories and in refining predictions of sentinel lakes' responses to global changes.

Quantifying impacts of titanium dioxide nanoparticles on natural assemblages of riverine phytobenthos and phytoplankton in an outdoor setting

Impacts of widespread release of engineered titanium dioxide nanoparticles (nTiO2) on freshwater phytoplankton and phytobenthic assemblages in the field, represents a significant knowledge gap. Using outdoor experiments, we quantified impacts of nTiO2 on phytoplankton and periphyton from UK rivers, applied at levels representative of environmentally realistic concentrations (0.05 mg/L) and hot spots of accumulation (5.0 mg/L). Addition of nTiO2 to river water led to rapid temporal size changes in homoagglomerates and many heteroaggregates of nTiO2 with cells in the phytoplankton, including green algae, pennate and centric diatoms, increasing settlement of some cells. Changes in phytoplankton composition were evident after 72-h resulting from a significant decline in the relative abundance of very small phytoplankton cells (1–3 μm), often accompanied by increases in centric diatoms at both concentrations. Significant changes detected in the composition of the phytobenthos after 12 days, following nTiO2 treatments, were not evident when using benthic diatoms alone after 56 days. A lack of inhibition in the maximum quantum yield (Fv/Fm) in phytobenthos after 72-h exposures contrasted with a significant inhibition in Fv/Fm in 75% of phytoplankton samples, the highest recorded in Rutile nTiO2 exposures at both concentrations of nTiO2. After 12 days, strong positive stimulatory responses were recorded in the maximum relative electron transport rate (rETRmax) and the maximum non-photochemical coefficient (NPQmax), in phytoplankton and phytobenthos samples exposed to the higher Anatase nTiO2 concentration, were not measured in Rutile exposed biota. Collectively, these results indicate that the Rutile phase of nTiO2 has more negative impacts on freshwater algae than the Anatase form, at specific time scales, and phytoplankton may be more impacted by nTiO2 than phytobenthos. We caution that repeated release of nTiO2, could lead to significant changes in riverine algal biomass and species composition, dependent on the phase and concentration of nTiO2.

An overview of Lepidoptera-host-parasitoid associations for southern Africa, including an illustrated report on 2 370 African Lepidoptera-host and 119 parasitoid-Lepidoptera associations

We present an overview of the known host associations of larval Lepidoptera for southern Africa, based on a database of 11 628 rearings, including all Caterpillar Rearing Group (CRG) records and other published records. Rearings per Lepidoptera family show some bias in the rearing effort towards the more conspicuous families, ectophagous groups and non-detritus-feeders but in general follow species diversity. Recorded Lepidoptera host associations per host family for southern Africa are shown. Data analyses revealed the following general trends: of the 20 most reared species 13 are polyphagous; Fabaceae are the most utilised plant family with 2 122 associations, followed by Asteraceae (600), Malvaceae (564) and Anacardiaceae (476); 98.8 % of hosts are vascular plants; and of the 19 most utilised host species 18 are common trees or shrubs. We discuss possible reasons behind these trends, particularly the high utilisation of Fabaceae and the widespread use of trees and shrubs as hosts. We compare recorded host species numbers with species diversity for the 19 most recorded host families and discuss possible reasons for the low utilisation of four plant families with an exceptionally low percentage of Lepidoptera host species / plant host species diversity. All Lepidoptera families for which more than 100 rearings have been recorded (21 families) utilise one (or two in the case of Pyralidae, Nolidae and Hesperiidae) plant family exponentially more than any of the other families, with resulting histograms forming hyperbolic curves, as are typical of distributions of taxonomic assemblages in nature. We calculate an exponential factor to quantify this phenomenon and show that for all 21 Lepidoptera families one host family is utilised 6–33 times more than the average use of other host families. In this paper, the larvae and adults of 953 African, mostly South African, Lepidoptera species reared by the CRG between January 2016 and June 2019 are illustrated together with pertinent host information. 119 Lepidoptera-parasitoid associations are reported, comprising seven hymenopteran families and one dipteran family. With the current data release, larval host association records are now available for 2 826 Lepidoptera species in the southern African subregion, covering about 25 % of the described fauna.